Typically, pH combination electrode assemblies utilize a glass frit as a porous member. The function of the glass frit is to separate a silver-silver chloride reference electrode immersed in a potassium chloride solution from the solution under test and yet allow electrical contact to be maintained between the electrode and the solution under test. Unfortunately, the glass frit frequently becomes clogged, for example, by using the electrode assembly to measure the pH of solutions containing particles. More commonly, the glass frit becomes clogged because the silver chloride on the silver-silver chloride electrode is much more soluble in potassium chloride solutions than in the usual solutions under test. As the potassium chloride solution containing dissolved silver chloride slowly flows through the glass frit and meets the solution under test, the dissolved silver chloride precipitates on the surface and/or the interior of the glass frit causing the glass frit to plug and disrupt electrical contact.
When the glass frit becomes plugged, it is possible to return the plug to working condition. For example, the electrode can be immersed in fresh potassium chloride solution which is then heated to near boiling to redissolve the precipitated silver chloride. However, unless one is very careful, this may result in the electrode assembly cracking and rendering it useless. Another method of unplugging the glass frit is to manually pick at the surface to dislodge precipitated silver chloride. However, picking at the surface may also render an electrode useless by destroying the glass frit unless exteme care is taken.
Various electrode assemblies comprising concentric cylindrical tubes and having a porous member providing communication between a solution to be tested and an internal electrolyte are disclosed in the following U.S. Patents: U.S. Pat. No. 3,476,671 (Petty); U.S. Pat. No. 4,390,406 (Kato et al.); U.S. Pat. No. 4,182,668 (Koshiishi et al.); U.S. Pat. No. 3,546,087 (Friconneau et al.); U.S. Pat. No. 4,012,308 (Jerrold-Jones et al.); U.S. Pat. No. 4,008,141 (Kotani et al.); U.S. Pat. No. 3,666,651 (Makabe); and U.S. Pat. No. 4,128,468 (Bukamier).